I managed all of them, but it was hard to see exactly what the allowed region was.

hmm...is this roughly what you see as the highlight area?

https://imgur.com/a/Lee1HRu

I wonder if the contrast isn't high enough for some users. Honestly, this is the first time I've gotten this feedback. Maybe try the dark fretboard option? It looks like this:

https://imgur.com/a/prLpyOq